Ionic liquids comprising heteraromatic anions

ABSTRACT

Some embodiments described herein relate to ionic liquids comprising an anion of a heteraromatic compound such as optionally substituted pyrrolide, optionally substituted pyrazolide, optionally substituted indolide, optionally substituted phospholide, or optionally substituted imidazolide. Methods and devices for gas separation or gas absorption related to these ionic liquids are also described herein.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.15/802,360, filed on Nov. 2, 2017, which is a divisional of U.S. patentapplication Ser. No. 13/505,730, filed on May 2, 2012, now U.S. Pat. No.9,951,008, issued on Apr. 24, 2018, which is a U.S. National Stage Entryunder 35 U.S.C. § 371 of International Patent Application No.PCT/US2010/055330, filed on Nov. 3, 2010, which claims priority to U.S.Provisional Patent Application No. 61/257,795, filed on Nov. 3, 2009,the entire contents of each of which are fully incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED R&D

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms ofDE-FC26-07NT43091 awarded by the United States Department of Energy.

BACKGROUND OF THE INVENTION Field of the Invention

The embodiments described herein relate to ionic liquids, such as ionicliquids used for separating or absorbing acidic gases.

Description of the Related Art

Ionic liquids are increasingly being used for a number of applications,such as for separation or purification of gases, or for otherapplications that involve the absorption of one or more gases. Ofparticular interest is the separation of acidic gases, such as oxides ofcarbon, nitrogen, or sulfur. These gases are common impurities in manygas mixtures, and are common air pollutants. For example, carbon captureand sequestration, the removal of carbon dioxide from combustionexhaust, or from air, to reduce the greenhouse effect on global warming,is of particular interest.

The atmospheric concentration of CO₂ has increased unabated since thedawn of the industrial revolution, due primarily to CO₂ emissions fromthe combustion of fossil fuels, and this growing carbon burden may havesignificant implications for the global climate. While the developmentof a carbon-neutral infrastructure may be a long-term solution to thisproblem, the increasing world demand for energy and the readyavailability of fossil fuels—in particular coal-make it highly likelythat fossil fuel combustion will continue to be a substantial fractionof the energy portfolio for the foreseeable future. In this environment,alternative approaches to managing CO₂ emissions become desirable.

For coal-fired power plants and other point source emitters,post-combustion carbon capture may be the most straightforward andpromising route to limiting CO₂ release, but practical carbon capturemay depend on the discovery of energy-efficient means of separating CO₂from the other gaseous components of a flue gas. For example a typical500 MW coal-fired power plant produces about 22 kmol s⁻¹ of flue gascontaining ˜15% CO₂ in N₂, O₂, H₂O and other trace gases at near ambienttemperature and pressure. Separating CO₂ from this stream may consumemore than 30% of the power of the plant using presently available amineabsorption technologies, far above the theoretical minimum work ofseparation.

Many ionic liquids may be unsuitable for industrial removal of carbondioxide and other acidic gases because they may become highly viscouswhen the acidic gas is absorbed, or because they may not be suitable forabsorption of the gas and the subsequent removal of the gas to store thegas and/or to recycle the ionic liquid. Therefore, there is a need foradditional ionic liquids which may improve one or more of theseproperties.

SUMMARY OF THE INVENTION

Some embodiments provide an ionic liquid comprising an anion selectedfrom: optionally substituted pyrrolide optionally, substitutedpyrazolide, optionally substituted indolide, optionally substitutedphospholide, or optionally substituted imidazolide.

Some embodiments provide an ionic liquid comprising an anion representedby a Formula 1 or Formula 2:

Some embodiments related to an anionic liquid comprising an anionrepresented by Formula A:

[Het··CO₂]⁻  (Formula A);

wherein ·· represents a covalent or non-covalent bonding interaction;and Het is optionally substituted heteroaryl.

Some embodiments relate to an ionic liquid comprising an anionrepresented by a Formula 7 or Formula 8:

In Formulas 1, 2, 7, and 8, X is N or P; A is N or CR³; E is N or CR⁴; Gis N or CR⁵; J is N or CR⁶; and R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, andR¹⁰ are independently halo, CN, CNO, NCO, NO₂, R¹¹, OR¹¹, SR¹¹, NR¹²R¹³,—YC(O)ZR¹¹, SO₂R¹¹, SO₃R¹¹, or SO₂NR¹²R¹³; Y is a single bond,optionally substituted C₁₋₆ hydrocarbyl, —N(R¹²)—, O, or S; Z is asingle bond, —N(R¹²)—, O, or S; each R¹¹ is H or optionally substitutedC₁₋₁₂ hydrocarbyl; and each R¹² and each R¹³ is independently H oroptionally substituted C₁₋₆ hydrocarbyl.

Some embodiments relate to a method of separating an acidic gas from amixture of gases, comprising: providing sufficient contact between themixture of gases and an ionic liquid described herein to allow at leasta portion of the acidic gas to be absorbed by the ionic liquid toprovide a purified gas; and collecting or diverting for use the purifiedgas and/or the acidic gas.

Some embodiments provide a method of separating an acidic gas from acombustion exhaust, comprising: providing an exhaust from combustion ofa carbon-based fuel, wherein the exhaust comprises the acidic gas; andproviding sufficient contact between the exhaust and an ionic liquiddescribed herein to allow at least a portion of the acidic gas to beabsorbed by the ionic liquid.

Some embodiments relate to a method of separating an acidic gas from amixture of gases, comprising: providing sufficient contact between themixture of gases and an ionic liquid described herein to allow at leasta portion of the acidic gas to be absorbed by the ionic liquid; andrecovering the acidic gas from the ionic liquid by applying at least oneof heat or reduced pressure to the ionic liquid.

Some embodiments relate to a method of cooling an enclosed volumecomprising compressing and expanding a gas comprising carbon dioxide inthe presence of an ionic liquid described herein.

Some embodiments provide a gas separation device comprising: a flowcomponent configured to provide a flow of a mixture of gases, whereinthe mixture of gases comprises an acidic gas; a separation component, influid communication with the flow component, configured to allow theflow of the mixture of gases to pass through the separation componentfrom the flow component; and the ionic liquid described herein, coupledto the separation component; wherein the device is configured to providesufficient contact between the mixture of gases and the ionic liquid toremove at least a portion of the acidic gas from the mixture of gases.

Some embodiments provide a combustion device comprising: a combustionvessel configured to contain a combustion reaction; an exhaustcomponent, in fluid communication with the combustion vessel, which isconfigured to allow exhaust from the combustion reaction to escape fromthe combustion vessel, wherein the exhaust comprises an acidic gas; andthe ionic liquid described herein, coupled to the exhaust component;wherein the device is configured to provide sufficient contact betweenthe exhaust and the ionic liquid to remove at least a portion of theacidic gas from the exhaust.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of an embodiment of a gas separation devicedescribed herein.

FIG. 2 is an example of an embodiment of a combustion device describedherein.

FIG. 3 is an example of an embodiment of a circulation system describedherein.

FIG. 4 is a plot of moles CO₂ absorbed/moles of ionic liquid against CO₂pressure in bars for some embodiments of the ionic liquids describedherein.

FIG. 5 is a plot of viscosity (cP) against temperature (° C.) for twocontrol ionic liquids comprising amino acid anions.

FIG. 6 is a plot of viscosity (cP) against temperature (° C.) for someembodiments of the ionic liquids described herein.

FIG. 7 is a plot of the infrared (IR) spectrum of an embodiment of ionicliquid described herein before and after reaction with carbon dioxide.

FIG. 8 is a plot of viscosity (cP) against temperature (° C.) for someembodiments of the ionic liquids described herein before exposure tocarbon dioxide and under 1 bar of carbon dioxide.

FIG. 9 is a plot of moles of carbon dioxide absorbed against carbondioxide pressure by an embodiment of ionic liquid disclosed herein atseveral different temperatures.

FIG. 10 is a plot of ln(k1) or ln(H) against 1/T (K−1) for an embodimentof ionic liquid disclosed herein.

FIG. 11 is a plot of moles of carbon dioxide absorbed against carbondioxide pressure by some embodiment of ionic liquid disclosed herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein the term “ionic liquid” has the ordinary meaningunderstood by a person of ordinary skill in the art. In someembodiments, the term “ionic liquid” may include a nonpolymeric saltthat is reasonably fluid under ambient conditions. The salt may comprisemonovalent or polyvalent anions or cations. In addition, the ionicliquid may be a single salt or a mixture of salts. In some embodiments,the ionic liquid is a liquid at a temperature in the range of from about1° C. to about 100° C. and at a pressure of about 1 atmosphere. It isappreciated that some ionic liquids may have melting points aboveambient temperatures or well below 1° C. However, ionic liquids may bedistinguished from conventional “molten salts”, such as sodium chloride,requiring excessive temperatures (e.g. greater than about 250° C.) toachieve a liquid phase. In some embodiments, ionic liquids may havenegligible vapor pressures under ambient conditions and may often formstable liquids at temperatures up to about 300° C. In some embodiments,ionic liquids may also have a wide range of miscibilities with organicsolvents and water. However, an ionic liquid is not necessarily solublein either organic solvents or water.

Unless otherwise indicated, when a chemical structural feature such ashydrocarbyl or an anion of heteroaryl moieties such as pyrazole, indole,phosphole, imidazole, is referred to as being “optionally substituted,”it is meant that the feature may have no substituents (i.e. beunsubstituted) or may have one or more substituents. A feature that is“substituted” has one or more substituents. The term “substituent” hasthe ordinary meaning known to one of ordinary skill in the art. In someembodiments, the substituent is an ordinary organic moiety known in theart, which may have a molecular weight (e.g. the sum of the atomicmasses of the atoms of the substituent) of less than: about 500 g/mol,about 300 g/mol, about 200 g/mol, about 100 g/mol, or about 50 g/mol. Insome embodiments, the substituent comprises: about 0-30, about 0-20,about 0-10, or about 0-5 carbon atoms; and about 0-30, about 0-20, about0-10, or about 0-5 heteroatoms independently selected from: N, O, S, P,Si, F, Cl, Br, I, and combinations thereof; provided that thesubstituent comprises at least one atom selected from: C, N, O, S, P,Si, F, Cl, Br, and I. Examples of substituents include, but are notlimited to, alkyl, alkenyl, alkynyl, carbazolyl, aryl, heteroaryl,heteroalicyclyl, aralkyl, heteroaralkyl, (heteroalicyclyl)alkyl,hydroxy, protected hydroxy, alkoxy, aryloxy, acyl, ester, mercapto,alkylthio, arylthio, cyano, halogen, carbonyl, thiocarbonyl, O-carbamyl,Ncarbamyl, Othiocarbamyl, Nthiocarbamyl, Camido, Namido, S-sulfonamido,Nsulfonamido, Ccarboxy, protected C-carboxy, Ocarboxy, isocyanato,thiocyanato, isothiocyanato, nitro, silyl, sulfenyl, sulfinyl, sulfonyl,haloalkyl, haloalkoxyl, trihalomethanesulfonyl,trihalomethanesulfonamido, and amino, including mono and disubstitutedamino groups, and the protected derivatives thereof. In someembodiments, two substituents may together form an aliphatic or anaromatic ring.

The term “heteroaryl” also has the meaning understood by a person ofordinary skill in the art, and in some embodiments, may refer to anaromatic ring which has one or more heteroatoms in the ring or ringsystem. Examples of “heteroaryl” may include, but are not limited to,pyridinyl, furyl, thienyl, oxazolyl, thiazolyl, imidazolyl, quinolinyl,benzofuranyl, benzothienyl, benzooxazolyl, benzothiazolyl,benzoimidazolyl, etc.

The parent ring structures associated with certain optionallysubstituted anionic ring systems are depicted below. The name of eachanion is shown below the structure of the anion. If the ring structureis substituted, a substituent may be present on any ring carbon atom.

As used herein, the term “halo” refers to a halogen, such as F, Cl, Br,or I.

As used herein, the term “hydrocarbyl” refers to a moiety composed ofcarbon and hydrogen. Hydrocarbyl includes alkyl, alkenyl, alkynyl, aryl,etc., and combinations thereof, and may be linear, branched, cyclic, ora combination thereof. Hydrocarbyl may be bonded to any other number ofmoieties (e.g. be bonded to 1 other group, such as —CH₃, —CH═CH₂, etc.;2 other groups, such as -phenyl-, —CαC—, etc.; or any number of othergroups) that the structure may bear, and in some embodiments, maycontain from one to thirty-five carbon atoms. Examples of hydrocarbylgroups include but are not limited to C₁ alkyl, C₂ alkyl, C₂ alkenyl, C₂alkynyl, C₃ alkyl, C₃ alkenyl, C₃ alkynyl, C₄ alkyl, C₄ alkenyl, C₄alkynyl, C₅ alkyl, C₅ alkenyl, C₅ alkynyl, C₆ alkyl, C₆ alkenyl, C₆alkynyl, phenyl, etc.

As used herein, the term “alkyl” refers to a moiety composed of carbonand hydrogen containing no double or triple bonds. Alkyl may be linear,branched, cyclic, or a combination thereof, may be bonded to any othernumber of moieties (e.g. be bonded to 1 other group, such as —CH₃, 2other groups, such as —CH₂—, or any number of other groups) that thestructure may bear, and in some embodiments, may contain from one tothirty-five carbon atoms. Examples of alkyl groups include but are notlimited to CH₃ (e.g. methyl), C₂H₅ (e.g. ethyl), C₃H₇ (e.g. propylisomers such as propyl, isopropyl, etc.), C₃H₆ (e.g. cyclopropyl), C₄H₉(e.g. butyl isomers) C₄H₈ (e.g. cyclobutyl isomers such as cyclobutyl,methylcyclopropyl, etc.), C₅H₁₁ (e.g. pentyl isomers), C₅H₁₀ (e.g.cyclopentyl isomers such as cyclopentyl, methylcyclobutyl,dimethylcyclopropyl, etc.) C₆H₁₃ (e.g. hexyl isomers), C₆H₁₂ (e.g.cyclohexyl isomers), C₇H₁₅ (e.g. heptyl isomers), C₇H₁₄ (e.g.cycloheptyl isomers), C₈H₁₇ (e.g. octyl isomers), C₈H₁₆ (e.g. cyclooctylisomers), C₉H₁₉ (e.g. nonyl isomers), C₉H₁₈ (e.g. cyclononyl isomers),C₁₀H₂₁ (e.g. decyl isomers), C₁₀H₂₀ (e.g. cyclodecyl isomers), C₁₁H₂₃(e.g. undecyl isomers), C₁₁H₂₂ (e.g. cycloundecyl isomers), C₁₂H₂₅ (e.g.dodecyl isomers), C₁₂H₂₄ (e.g. cyclododecyl isomers), C₁₃H₂₇ (e.g.tridecyl isomers), C₁₃H₂₆ (e.g. cyclotridecyl isomers), and the like.

An expression such as “C₁₋₁₂” (e.g. “C₁₋₁₂ hydrocarbyl”) refers to thenumber of carbon atoms in a moiety, and similar expressions have similarmeanings. Generally, an expression such as “C₁₋₁₂” refers only to thenumber of carbon atoms in a parent group, and does not characterize orlimit the substituents in any way. For a hydrocarbyl moiety, the parentgroup includes all carbon atoms which are directly bonded to anothercarbon atom of the parent group (except for C₁ hydrocarbyl). Forexample, the carbon atoms which are counted are numbered in the moietiesbelow:

As used herein the term “bonding interaction” has the ordinary meaningunderstood by a person of ordinary skill in the art. In someembodiments, the term “bonding interaction” may include traditionalcovalent interactions or covalent bonds, or non-covalent interactionssuch as electron donor-acceptor interactions, dipole-dipoleinteractions, induced dipole-dipole interactions, hydrogen bonding, etc.

As used herein the term “acidic gas” has the ordinary meaning understoodby a person of ordinary skill in the art. In some embodiments, the term“acidic gas” may include any gas which is acidic. In some embodiments,the acid gas may be more acidic than water, or may cause water to have apH of less than about 7 when the gas is dissolved in water. In someembodiments, the acidic gas is an oxide of carbon such as carbondioxide, carbon monoxide, and the like. In some embodiments, the acidicgas is an oxide of sulfur such as SO, SO₂, SO₃, and the like. In someembodiments, the acidic gas is an oxide of nitrogen such as N₂O, NO,NO₂, and the like.

As used herein, the phrase “a method of separating an acidic gas from amixture of gases” should be construed to include any method whichseparates a composition comprising the acidic gas from the mixture ofgases such that the purified gas comprises less of the acidic gas thanit did prior to separation. In some embodiments, the method reduces theconcentration or partial pressure of the acidic gas in the mixture ofgases by at least about 20%, about 50%, about 90%, or about 99%.

As used herein the term “combustion product” has the ordinary meaningunderstood by a person of ordinary skill in the art. In someembodiments, the term “combustion product” may include any gases whichare evolved as a product of a combustion reaction, such as a reaction ofa carbon-based fuel including coal, a petroleum product, natural gas,etc. Examples of combustion products include oxides of elements such ascarbon, sulfur, nitrogen, and the like.

Parameters such as the selectivity of absorption of an acidic gas, theability to control absorption of an acidic gas, the reversibility of theabsorption, and other related properties may be varied according to theapplication. Structural variations in ionic liquids may allow one ormore of these parameters to be adjusted.

Some embodiments provide an ionic liquid comprising: optionallysubstituted pyrrolide, optionally substituted pyrazolide, optionallysubstituted indolide, optionally substituted phospholide, or optionallysubstituted imidazolide. In some embodiments, the ionic liquid comprisespyrrolide having 0, 1, or 2 substituents. In some embodiments, the ionicliquid comprises pyrazolide having 0 or 1 substituents. In someembodiments, the ionic liquid comprises indolide having 0, 1, 2, or 3substituents.

In some embodiments, the anion is represented by Formula 1 or Formula 2,as depicted above.

With respect to Formula 1, X is N or P, A is N or CR³, E is N or CR⁴, Gis N or CR⁵, and J is N or CR⁶. Thus, some embodiments relate to ionicliquids comprising an anion represented by Formula 3, Formula 4, Formula5, or Formula 6.

With respect to Formulas 1-6, although the formal charge of thesestructures may lie on an N or a P atom, it is believed that the actualdistribution of the negative charge may include other atoms of thestructure. Thus, the actual negative charge can be in any position or onany atom or combinations of atoms on the ion and still be within thescope of these formulas.

In some embodiments, an anion of a heteraromatic ring may form acovalent or non-covalent complex with carbon dioxide, such as an anionrepresented by Formula A, wherein ·· represents a covalent ornon-covalent bonding interaction; and Het is optionally substitutedheteroaryl. Some embodiments provide an ionic liquid comprising ananionic complex of optionally substituted pyrrolide and CO₂, optionallysubstituted pyrazolide and CO₂, optionally substituted indolide and CO₂,optionally substituted phospholide and CO₂, or optionally substitutedimidazolide and CO₂.

In some embodiments, an anion of Formula 1, Formula 2, Formula 3,Formula 4, Formula 5, or Formula 6 may react with CO₂ to provide ananion represented by Formula 7, Formula 8, Formula 9, Formula 10,Formula 11, or Formula 12. Thus, some embodiments relate to ionicliquids comprising anions represented by any of Formulas 7-8, depictedabove, and 9-12.

Reaction with other acidic gases, such as SO₂, would result in compoundssimilar to those in Formulas 7-12 where the CO₂ group is replaced by agroup formed by the corresponding acid gas molecule.

With respect to Formulas 7-12, although the formal charge of thesestructures may lie on one or both of the oxygen atoms, it is believedthat the actual distribution of the negative charge may include otheratoms of the structure. Thus, the actual negative charge can be in anyposition or on any atom or combinations of atoms on the ion and still bewithin the scope of these formulas.

With respect to Formulas 1-12, R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, andR¹⁰ are independently halo, CN, CNO, NCO, NO₂, R¹¹, OR¹¹, SR¹¹, NR¹²R¹³,—YC(O)ZR¹¹, SO₂R¹¹, SO₃R¹¹, or SO₂NR¹²R¹³. Any of these groups onadjacent carbon atoms, e.g. R¹ and R², R² and R³, R³ and R⁴, R⁵ and R⁶,R⁷ and R⁸, R⁸ and R⁹, R⁹ and R¹⁰, etc., may together form an aliphaticor an aromatic ring. For example, any of these pairs may, together withthe parent ring atoms, form an additional optionally substituted phenylring, an optionally substituted cyclopentenyl ring, an optionallysubstituted hexenyl ring, an optionally substitute pyrrole ring, and thelike. Thus, in some embodiments, the anion may comprise optionallysubstituted benzopyrrolide, optionally substituted benzopyrazolide,optionally substituted optionally substituted benzophospholide, oroptionally substituted benzoimidazolide.

With respect to R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰, Y is asingle bond, optionally substituted C₁₋₆ hydrocarbyl, —N(R¹²)—, O, or S;Z is a single bond, —N(R¹²)—, O, or S; each R¹¹ is H or optionallysubstituted C₁₋₁₂ hydrocarbyl; and each R¹² and each R¹³ isindependently H or optionally substituted C₁₋₆ hydrocarbyl.

With respect to —YC(O)ZR¹¹, if Y is a single bond, the moiety may berepresented by —C(O)ZR¹¹. In some embodiments, Y is optionallysubstituted C₁₋₆ hydrocarbyl, such as C₁₋₆ alkyl including —(CH₂)_(n)—and —C_(n)H_(2n)—, wherein n is 1, 2, 3, 4, 5, or 6. Also with respectto —YC(O)ZR¹¹, if Z is a single bond, the moiety may be represented by—YC(O)R¹¹.

With respect to R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰, R¹¹ may beH so that any of R¹⁻¹⁰ may independently be H, OH, SH, —YC(O)ZH, SO₂H,or SO₃H. In some embodiments, R¹¹ may be optionally substituted C₁₋₁₂hydrocarbyl, such as C₁₋₁₂ alkyl, optionally substituted phenyl,optionally substituted naphthyl, or optionally substituted biphenyl. Insome embodiments, each R¹¹ is independently H or C₁₋₃ alkyl.

With respect to R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, and R¹⁰, R¹¹, eachR¹² and each R¹³ may be independently H so that any of R¹⁻¹⁰ mayindependently be NH₂, NHR¹², or SO₂NHR¹². In some embodiments, R¹² andR¹³ may independently be may be optionally substituted C₁₋₆ hydrocarbyl,such as C₁₋₆ alkyl or optionally substituted phenyl. In someembodiments, each R¹² and each R¹³ is independently H or optionallysubstituted C₁₋₃ alkyl.

With respect to any of the combinations described above related toFormulas 1-12, in some embodiments, R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹,and R¹⁰ are independently halo, CN, CNO, NCO, NO₂, R¹¹, OR¹¹, SR¹¹,NR¹²R¹³, —C(O)R¹¹, —C(O)OR¹¹, —C(O)NR¹¹R¹², —OC(O)R¹¹, —OC(O)OR¹¹,—OC(O)NR¹¹R¹², —N(R¹²)C(O)R¹¹, —N(R¹²)C(O)OR¹¹, —N(R¹²)C(O)SR¹¹,—N(R¹²)C(O)NR¹¹R¹², —SO₂R¹¹, —SO₃R¹¹, or —SO²NR¹¹R¹². In someembodiments, R¹ is F, CF₃, CHF₂, CH₂F, CN, CO₂CH₃, acetyl, ortrichloroacetyl. In some embodiments, R² is F, CF₃, CHF₂, CH₂F, CN,CO₂CH₃, acetyl, or trichloroacetyl.

In some embodiments, the ionic liquid comprises at least one anionselected from: 2-fluoropyrrolide, 2-trifluoromethylpyrrolide,1-fluoromethylpyrrolide, 2-cyanopyrrolide, 1-difluoromethylpyrrolide,1-fluoropyrrolide, 1-trifluoromethylpyrazolide, 1-cyanopyrrolide,1-methylesterpyrrolide, 1-trifluoromethylpyrrolide, 1-acetylpyrrolide,and 1-trichloroacetylpyrrolide.

These anions may be in ionic liquids which may be used for removing orseparating an acidic gas from a mixture of gases. In some embodiments,the ionic liquid may be mixed with one or more addition diluents,wherein the ionic liquid acts as the active ingredient for removing orseparating the acidic gas from a mixture of gases. In many of theseuses, it may be desirable to first remove the acidic gas from themixture of gases, and then remove the acidic gas from the ionic liquidso that the acidic gas may be collected or used and/or so that the ionicliquid may be reused. Thus, in many embodiments, the particular use mayrequire the strength of the binding interaction between the ionic liquidand the acidic gases to be tuned to optimize the conditions under whichthe acidic gas is taken up and released by the ionic liquid.

While not limiting any embodiment, it is believed that the bindingstrength between CO₂ (or other acidic gas) and the ionic liquid may betuned by varying the substituents on the ionic liquid. Generally, it isbelieved that an electron-withdrawing substituent on an anion of aheteroaryl ring can reduce strength of the binding interaction beweenthe ionic liquid and CO₂. This may allow easier recovery of CO₂ after ithas been absorbed by the ionic liquid. Thus, in some embodiments, theanion of a heteroaryl ring may have at least one electron-withdrawingsubstituent. Examples of electron withdrawing substituents may include,but are not limited to, F, CF₃, CHF₂, CH₂F, CN, CO₂CH₃ or othercarboxyalkyl esters, acetyl or other acyl groups, trichloroacetyl, etc.Conversely, it is also believed than an electron-donating substituent onan anion of a heteroaryl ring can increase the strength of the bindinginteraction on an anion of a heteroaryl ring. This may provide easierabsorption of CO₂ (or other acidic gas) by the ionic liquid. It is alsobelieved that a substituent on the carbon immediately adjacent to aheteratom of the anion of a heteroaryl ring has a greater effect on thebinding strength than a substituent on a carbon which is more remote.For example, with respect to Formula 3, an electron withdrawing R¹ willreduce the strength of the binding to CO₂ more than an electronwithdrawing R². Thus, the binding strength may be tuned by varying thenumber, position, and nature of the substituents on the anion usingthese principles.

In some embodiments, the cations present in the ionic liquid may beselected to tune properties of the ionic liquid such as decompositiontemperature, density, viscosity, melting point, heat capacity, and thelike. Furthermore, in some embodiments, the cation may also be selectedto be chemically active towards carbon dioxide. The cation present inthe ionic liquid can be a single species or a plurality of differentspecies. Both of these embodiments are intended to be embraced, unlessotherwise specified, by the use of the singular expression “cation.” Thecations of the ionic liquid include organic and inorganic cations.Examples of cations include quaternary nitrogen-containing cations,phosphonium cations, and sulfonium cations. Suitable cations includethose disclosed in U.S. Pat. No. 7,053,232 and US Publication No.2005/0131118, the disclosures of which are hereby incorporated byreference in their entireties.

Examples of quaternary nitrogen-containing cations include, but are notlimited to, cyclic, aliphatic, and aromatic quaternarynitrogen-containing cations such as n-alkyl pyridinium, a dialkylpyrrolidinium, a dialkyl imidazolium, or an alkylammonium of the formulaR′_(4-X)NH_(X) wherein X is 0-3 and each R′ is independently an alkylgroup having 1 to 18 carbon atoms. In some embodiments, unsymmetricalcations may provide lower melting temperatures. Examples of phosphoniumcations include, but are not limited to, cyclic, aliphatic, and aromaticphosphonium cations. For example, the phosphonium cations include thoseof the formula R″_(4-X)PH_(X) wherein X is 0-3, and each R″ is an alkylor aryl group such as an alkyl group having 1 to 18 carbon atoms or aphenyl group. Examples of sulfonium cations include, but are not limitedto cyclic, aliphatic, and aromatic sulfonium cations. For example, thesulfonium cations include those of the formula R′″_(3-X)SH_(X) wherein Xis 0-2 and each R′″ is an alkyl or aryl group such as an alkyl grouphaving 1 to 18 carbon atoms or a phenyl group. Additional more specificexamples may include, but are not limited to, ammonium, imidazolium,phosphonium, 1-butyl-3-methylimidazolium, 1-decyl-3-methylimidazolium,1-dodecyl-3-methylimidazolium, 1-ethyl-3-butyl imidazolium,1-hexyl-3-methylimidazolium, 1-hexylpyridinium, 1-methy-3-butylimidazolium, 1-methy-3-decyl imidazolium, 1-methy-3-dodecyl imidazolium,1-methy-3-ethyl imidazolium, 1-methy-3-hexadecyl imidazolium,1-methy-3-hexyl imidazolium, 1-methy-3-octadecyl imidazolium,1-methy-3-octyl imidazolium, 1-methy-3-propyl imidazolium,1-octyl-3-methylimidazolium, 1-octylpyridinium, benzyl pyridinium,N-butyl pyridinium, ethyl pyridinium, and ethylene pyridinium. Otherexamples of suitable cations are known in the art. For example,US2006/0197053, US2008/0028777, and US2007/0144186, all of which areincorporated by reference in their entireties, describe a number ofsuitable cations, and any of these cations may be used with an aniondescribed herein.

The ionic liquids described herein may be used in a number of methodsrelated to gas separation, purification, or other uses related to theuptake of an acidic gas by an ionic liquid.

Ionic liquids have relatively low vapor pressures. Thus, under someconditions, they are not volatilized to a significant extent into apurified gas stream. Their low vapor pressure minimizes loss ofabsorbing material during use and provides a simple mechanism forregeneration, such as by heating, distillation, evacuation, and/or byextraction with a supercritical fluid.

For example, one embodiment provides a method of separating an acidicgas from a mixture of gases, comprising: providing sufficient contactbetween the mixture of gases and an ionic liquid described herein toallow at least a portion of the acidic gas to be absorbed by the ionicliquid to provide a purified gas; and collecting or diverting for usethe purified gas and/or the acidic gas. In some embodiments, the mixtureof gas comprises a combustion product or a combustion exhaust, such as acombustion product of a fossil fuel or other carbon-based fuel such asoil, gasoline, petroleum products, natural gas, other hydrocarbons,coal, methanol, ethanol, other alcohols, and the like. In someembodiments, the acidic gas is an oxide of carbon, an oxide of sulfur,or an oxide of nitrogen. In some embodiments, the acidic gas may be CO,CO₂, SO, SO₂, SO₃, N₂O, NO, NO₂, etc.

When sufficient contact is provided between the mixture of gases and theionic liquid, the acidic gas is absorbed by the ionic liquid to providea purified gas. Absorption of the acidic gas by the ionic liquidincludes absorption without reaction, or it may include absorbing theacidic gas in a manner that causes the acidic gas to decompose, or reactwith the ionic liquid or another solute in the ionic liquid. Forexample, absorption may provide anions such as those depicted inFormulas 7-12. In some embodiments, the ionic liquids described hereinmay have the advantage that the viscosity of the liquid remainsrelatively low as CO₂ is absorbed by the ionic liquid. In someembodiments, an ionic liquid may have a viscosity less than about 10,000cP, about 5,000 cP, or about 1,000 cP at a temperature of about 20° C.and under a CO₂ pressure of about 1 bar. In some embodiments, an ionicliquid may have a viscosity less than about 1,000 cP, about 500 cP, orabout 200 cP at a temperature of about 50° C. and under a CO₂ pressureof about 1 bar. In some embodiments, exposure of the ionic liquid to aCO₂ pressure of about 1 bar may increase the viscosity by less thanabout 10 times, about 5 times, or about 2 times as compared to theviscosity of the ionic liquid before exposure to the CO₂.

The temperature, pressure, and time of absorption may vary. In someembodiments, the temperature may be about 0° C., about 10° C., about 20°C., or about 50° C., up to about 100° C., about 150° C., about 200° C.,or about 300° C., including from about 0° C. to about 300° C., fromabout 0° C. to about 100° C., and other ranges including and bordered bythe preceding values. In some embodiments, the pressure may be in therange of about 10⁻⁴ bar to about 10 bar, 10⁻⁴ bar to about 1.0 bar, orabout 10⁻⁴ bar to about 0.1 bar and also includes pressures between suchvalues and ranges including and bordered by the preceding values. Insome embodiments, the contacting of gas and ionic liquid material may becarried out for about 0.1 s to 100 hr, 1 s hr to 10 hr, or 10 s hr toabout 1 hr, about 1 min to about 10 hr, or about 1 min to about 5 hr andalso includes times between such values and ranges including andbordered by the preceding values.

In some embodiments, the acidic gas, such as CO₂, may be removed fromthe ionic liquid by heating the ionic liquid and/or applying reducedpressure to the liquid. For example, that amount of acidic gas that someionic liquids can absorb may substantially decrease as temperature isincreased. Thus, increasing the temperature of an ionic liquid withabsorbed acidic gas may cause a substantial amount of the acidic gas tobe released from the ionic liquid. In some embodiments, the ionic liquidmay be heated to a temperature of at least about 40° C., about 50° C.,or about 100° C., up to about 150° C., about 200° C., or about 300° C.

Similarly, the amount of acidic gas that some ionic liquids can absorbmay substantially decrease as pressure is reduced. Thus, reducing thepressure of an ionic liquid with absorbed acidic gas may cause asubstantial amount of the acidic gas to be released from the ionicliquid. In some embodiments, the pressure may be in the range of about10⁻⁴ bar to about 10 bar, 10⁻⁴ bar to about 1.0 bar, or about 10⁻⁴ barto about 0.1 bar.

In some embodiments, the heating and/or applying reduced pressure may becarried out for about 0.1 s to 100 hr, 1 s to 10 hr, or 10 s to about 1hr, about 1 min to about 10 hr, or about 1 min to about 5 hr. Thus, theionic liquid may be reused.

In some embodiments, once the acidic gas is absorbed, the acidic gasand/or the purified gas mixture may be collected or diverted for use.The acidic gas may be collected for storage or use by applying reducedpressure and/or heat to the ionic liquid, as described above. In someembodiments, CO₂ may be collected and stored, such as in carbon captureand sequestration. In some embodiments, an acidic gas may be absorbedand the purified gas mixture may be diverted for a use. For example, themixture of gases may be natural gas containing impurities such as waterand carbon dioxide and sulfur-containing compounds. After the carbondioxide, water and sulfur compounds are absorbed by the ionic liquid,the purified natural gas may be diverted for various industrial uses. Inanother example, in the first step of producing purified nitrogen fromair, impurities such as water and carbon dioxide may be removed using amethod or an ionic liquid described herein. Subsequent separation of theoxygen and nitrogen may be performed by cryogenic distillation.

FIG. 1 is a schematic diagram of an embodiment of a gas separationdevice. A flow component 10 is provided which is configured to provide aflow of a mixture of gases 15. The flow component 10 may be anycomponent which is capable of providing a flow of gas or a mixture ofgases such as a pump, a pressurized container, a combustion vessel, etc.The flow component is in fluid communication with a separation component20, and the separation component is configured to allow a flow of amixture of gases to pass through the separation component 20 from theflow component 10. In some embodiments, the flow of the mixture of gasesmay pass directly from the flow component 10 through the separationcomponent 20. However, in other embodiments, the flow may pass throughother components of the device after exiting the flow component 10 andbefore passing through the separation component 20. An ionic liquid 30is coupled to the separation component 20, and the device is configuredto provide sufficient contact between the mixture of gases and the ionicliquid to remove at least a portion of the acidic gas from the mixtureof gases. FIG. 2 is a schematic diagram of an embodiment of a combustiondevice. The combustion device comprises a combustion vessel 40configured to contain a combustion reaction 50. An exhaust component 60,is in fluid communication with the combustion vessel 40, and isconfigured to allow exhaust 70 from the combustion vessel 40 to escapefrom the combustion vessel 40. The exhaust 70 may be a mixture of gasescomprising an acidic gas. An ionic liquid 30 is coupled to the exhaustcomponent 60, and the device is configured to provide sufficient contactbetween the exhaust 70 and the ionic liquid 30 to remove at least aportion of the acidic gas from the exhaust.

In some embodiments, the devices depicted by FIG. 1 and FIG. 2 mayfurther comprise a circulation system, as depicted in FIG. 3. Theabsorption unit 35, such as the separation component 20 of FIG. 1 or theexhaust component 70 of FIG. 2, may be coupled to a circulation system37 which circulates CO₂-rich ionic liquid 130 to a desorption unit 39,such as a high temperature stripper, which removes the acidic gas fromthe ionic liquid by a method described above. CO₂-poor ionic liquid 230is then circulated from the desorption unit 39 to the absorption unit35.

There are many ways that the ionic liquid 30 may be coupled to theseparation component 20 or the exhaust component 70 so that the deviceis configured to provide sufficient contact is provided between themixture of gases and the ionic liquid to effect the desired separation.For example, the ionic liquid may coat at least part of an interiorsurface of the separation component 20 or the exhaust component 70. Insome embodiments, a flow of the ionic liquid on the interior surface maybe provided. In some embodiments, a particulate solid substrate coatedwith the ionic liquid may be coupled to the separation component or theexhaust component to increase the surface area of the ionic liquid incontact with the mixture of gases. The ionic liquid may be part of theseparation component 20 or the exhaust component 30, or may be aseparate feature which is attached or otherwise coupled to, and/or influid communication with, the separation component 20 or the exhaustcomponent 30. In some embodiments, the process may be selected or thedevice may be configured to promote intimate mixing of the liquid ioniccompound with the source gas. In some embodiments, sufficient contactmay be provided by allowing the contact to occur for a time sufficientto allow significant removal of acidic gas. Thus, in some embodiments,systems maximizing surface area contact may be used. For example, insome embodiments, sufficient contact may be provided by permeationthrough a supported liquid membrane, or by use of conventional liquidabsorbers, such as counter-current liquid absorbers and the like.

Supported liquid membranes comprise a solvent such as an ionic liquidcontained within the pores of a solid microporous support, such as aceramic, metal, or polymeric support. In some embodiments, supportedliquid membranes fabricated from supports such as ceramics, metals, andcertain heat stable polymers may be used in higher than ambienttemperature operations. In some embodiments, such higher temperatureoperations may effect a more rapid separation, requiring less contacttime. In addition, these higher temperature operations may also be aconsequence of the process configuration, such as configurationsrequiring purification of high temperature exhaust gases or other gasesexiting high temperature operations. Supported liquid membranes suitablefor purifying high temperature gases may obviate the need to pre-coolsuch gases before contact with the supported liquid membrane.Microporous supports suitable for use in the present methods and theirmethods of preparation are well known in the art (see, for example, U.S.Pat. Nos. 3,426,754; 3,801,404; 3,839,516; 3,843,761; 3,843,762;3,920,785; 4,055,696; 4,255,376; 4,257,997; 4,359,510; 4,405,688 and4,438,185, the disclosures of which are hereby incorporated byreference).

In some embodiments, the ionic liquid may be used in a conventionalgas/liquid absorption unit-based system comprising a fixed bed. Suchsystems can be operated in batch mode or continuous flow mode. In atypical batch mode configuration, the ionic liquid may be introducedinto a vessel followed by introduction of the mixture of gases. After aprescribed residence time, the resulting gas is removed, leaving behindthe acidic gas dissolved in the ionic liquid. The acidic gas can becollected by heating and/or reduced pressure treatment. To increasecontact between the ionic liquid and the mixture of gases, the ionicliquid can be coated on a solid support, such as glass beads, and thelike, to increase the surface area of the ionic liquid capable ofcontacting the mixture of gases.

In some embodiments, the ionic liquid may be contacted with the mixtureof gases in a flow apparatus. The above batch processes may be adaptedfor flow where the flow rate through the vessel correlates to theresidence time of contact and may be suitably chosen to afford aneffluent stream with the desired purification tolerance. When the ionicliquid has sufficiently absorbed the acidic gas the acidic gas can becollected by heating and/or vacuum as described.

In some embodiments, to promote intimate mixing, gas/liquid absorptionunits also may be operated in a dual flow mode. In some embodiments, thedual flow can be co-current or counter-current. In these embodiments,the mixture of gases and the ionic liquid may flow through apurification unit contemporaneously. In either the co-current or thecounter-current aspects, the acidic gas may be removed from thecontacted ionic liquid prior to reintroduction to the purification unit.

Some embodiments provide a method of cooling an enclosed volumecomprising compressing and expanding a gas, such as one comprisingcarbon dioxide, in the presence of an ionic liquid described herein. Insome embodiments, the gas may be the refrigerant, and an ionic liquiddescribed herein may be an absorbent, as described with respect to themethods and devices of US2007/0144186 and US2006/0197053, which areexpressly incorporated by reference herein in their entireties.Furthermore, any device described in those documents may be adapted foruse herein.

Synthetic Methods

While many methods may be used to prepare the ionic liquids describedherein, one convenient method is to obtain the neutral form of theanion, e.g. a heteraromatic compound such as a substituted pyrrole, anoptionally substituted pyrazole, an optionally substituted indole, anoptionally substituted phosphole, an optionally substituted imidazole,etc., and to deprotonate the neutral form with a hydroxide salt of aphosphonium ion. Many neutral heteraromatic compounds may be obtainedfrom commercial sources (e.g., Pyrrole-2-carbonitrile (96%),2-acetylpyrrole (98%), methyl 2-pyrrolecarboxylate (97%),3-(trifluoromethyl)pyrazole (99%)) can be purchased from Sigma-Aldrich)or prepared using methods known in the art, such as by nucleophilic orelectrophilic aromatic substitution methods. The phosphonium hydroxidesmay be prepared by a number of methods, known in the art, such as byanion exchange of a phosphonium halide. Many phosphonium halides may beobtained from commercial sources (e.g., CYPHOS IL 102, [P₆₆₆₁₄][Br])from Cytec Industries, Inc).

Example 1 Synthesis of Trihexyl(Tetradecyl)Phosphonium Cyanopyrrolide,[P₆₆₆₁₄][2-CNpyr]:

The synthesis of [P₆₆₆₁₄][2-CNpyr] is typical of a method that may beused to prepare many of the ionic liquids described herein. Thephosphonium salt, [P₆₆₆₁₄][Br], is diluted with methanol (2M) and thentransformed to [P₆₆₆₁₄][OH] by adding 2:1 molar equivalents of an anionexchange resin (e.g., DOWEX SBR LC NG (OH) ion exchange resin from DowChemical Company) in three batches, allowing 12 hrs in between. Theresin is pretreated with 1:1 volume equivalent of methanol by rinsing 3times at room temperature since the resin is unstable at temperaturesabove 60° C. During the ion exchange step, the mixture is stirred gentlywithout a stir bar (which could deform the resin beads). Afterfiltration to remove the resin, the [P₆₆₆₁₄][OH] solution is mixed witha 1:1 molar equivalent of pyrrole-2-carbonitrile and stirred for 12 hrs.This produces the [P₆₆₆₁₄][2-CNpyr] and water. Excess methanol isremoved by rotary evaporator at 50° C. and the [P₆₆₆₁₄][2-CNpyr]solution is further dried under vacuum for 72 hrs at 50° C. Similarprocedures are followed with other heterocyclic anion precursors to makeother ionic liquids described herein.

Example 2

Density functional theory (B3LYP/6-311G++(d,p)) was used as an indicatorof the reaction energy between carbon dioxide and the anion of the ionicliquid for different substituents. In these calculations, the structuresand energies of the unreacted anion, reacted anion, and CO₂ werecombined to estimate the overall reaction energy (ΔE).

Table 1 shows representative results for an unsubstituted pyrrolideanion (e.g. the anion of pyrrole) and various monosubstituted pyrrolideanions. The calculated reaction energy for the parent pyrrolide is −99kJ mol⁻¹. While not limiting any embodiment, it is believed thatelectronegative substituents withdraw charge from the pyrrolide,decrease the anion —CO₂ bond energy, and increase the reaction energy.For instance, placing fluorine in the R₂ position (2-fluoropyrrolide) orR₁ (1-fluoropyrrolide) positions increases the reaction energy to −89and −46 kJ mol⁻¹, respectively. As can be seen from the Table, othersubstituents (CN, CH_(X)F_(3-x), . . . ) have similar effects. It isbelieved that similar trends will be seen with anions or other heterarylrings, such as the imidazolides or pyrazolides.

TABLE 1 Reactant B3LYP Calorimetric Reactant name Structure ΔH (kJ/mol)ΔH (kJ/mol) Pyrrolide

−99 2-fluoro- pyrrolide

−89 2-trifluoro- methylpyrrolide

−67 1-fluoromethyl- pyrrolide

−67 2-cyano- pyrrolide

−59 1-difluoro- methyl- pyrrolide

−58 1-fluoro- pyrrolide

−46 1-trifluoro- methyl- pyrazolide

−44 −45 1-cyano- pyrrolide

−35 −48 1-methylester- pyrrolide −33 −10 1-trifluoro- methylpyrrolide

−32 1-acetyl- pyrrolide

−28 1-trichloro- acetylpyrrolide

1

It was believed that a lower, i.e. more negative, calculated ΔE maycorrespond to stronger binding or more absorption of carbon dioxide.Thus, it is believed that the calculated ΔE may provide a good model fortuning CO₂ absorption based upon the substituents used on the anionicring. To test this model, absorption isotherms (moles of CO₂/moles ionicliquid plotted against CO₂ pressure) were measured for three differentsubstituted pyrrolides. FIG. 4 provides a plot of these isotherms. Theplots show that the weakest binding, seen as a smaller slope of molesCO₂/moles ionic liquid versus CO₂ pressure, was observed formethylesterpyrrolide, which had the highest calculated ΔE (−33 kJ/mole).It was also seen that cyanopyrrolide, which had an intermediatecalculated ΔE (−35 kJ/mole), had intermediate binding. Finally,trifluoromethylpyrazolide, which had the lowest calculated ΔF (−44kJ/mole), had the strongest binding. Also shown on the graph is[P₆₆₆₁₄][Prolinate], an amino acid based ionic liquid control. It has amuch lower calculated ΔE (−71 kJ/mol from calculations and −77 kJ/molfrom calorimetry), consistent with the steep slope.

FIG. 4 also shows that the isotherms maximize near 1:1 mole ratio, whichmay be evidence that the anion reacts with CO₂ to obtain a CO₂substituted ion as depicted in Formulas 7-12.

Calorimetric studies of CO₂ absorption enthalpies were also performedfor the same three compounds, and results are shown in Table 1. Again,the results qualitatively agree with the experimental results depictedin FIG. 3, especially when uncertainties in both the computed numbersand the experiments are considered.

Similar studies may also be done using acidic gases other than CO₂.

Example 3

The viscosity of control ionic liquids comprising glycinate or lysinateunder 0 bar and 1 bar pressure of CO₂ is plotted against temperature inFIG. 5. Like most amino acid based ionic liquids (e.g. those publishedpreviously in the literature), these ionic liquids exhibit a largeincrease in viscosity when the CO₂ pressure is increased from 0 bar to 1bar. While not limiting any embodiment, these large viscosity increasesmay make use of these ionic liquids to separate of CO₂ from other gasesor absorb CO₂ more difficult in industrial processes.

Example 4

The viscosity of [P₆₆₆₁₄][methylesterpyrrolide],[P₆₆₆₁₄][cyanopyrrolide], and [P₆₆₆₁₄][trifluoromethylpyrazolide] with 0bar pressure of CO₂ and with 1 bar pressure of CO₂ is plotted in FIG. 6.The viscosity increases for these compounds is much smaller than thoseobserved for the control ionic liquids.

Example 5

Trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P₆₆₆₁₄][2-CNpyr]), astable ionic liquid, was synthesized as described in Example 1 above.The [P₆₆₆₁₄][2-CNpyr] was reacted with CO₂ at a pressure of about 1 bar.FIG. 7 is a plot of the infrared (IR) spectrum of the ionic liquidbefore and after reaction. The unreacted ionic liquid has a feature atabout 2183 cm⁻¹, which may be characteristic of a CN group. Afterreaction, the CN band shifts to about 2220 cm⁻¹, and prominent peaksappear at about 1728 cm⁻¹ and about 1200 cm⁻¹ to about 1300 cm⁻¹ whichmay indicate —NCOO⁻ stretches. At about 3 bar pressure, a band due tophysically dissolved CO₂ appeared between about 2470 cm⁻¹ and 2300 cm⁻¹.Application of a vacuum caused the IR spectrum to be restored to thatobserved before reaction.

FIG. 8 shows that the viscosity of [P₆₆₆₁₄][2-CNpyr] under 1 bar CO₂ isabout the same as unreacted [P₆₆₆₁₄][2-CNpyr] for at least thetemperature range of about 10° C. to about 70° C. FIG. 9 shows theisotherms of [P₆₆₆₁₄][2-CNpyr] at various temperatures. The steepinitial slopes may reflect chemical reaction between CO₂ and the[P₆₆₆₁₄][2-CNpyr]. The gradual slopes at higher pressure may reflect thecontribution of weaker physical absorption. The uptake approaches 1 moleCO₂ per mole [P₆₆₆₁₄][2-CNpyr] at the highest pressures and lowesttemperatures shown, consistent with a 1:1 stoichiometry. At highertemperatures, the uptake goes above 1 mole CO₂ per mole[P₆₆₆₁₄][2-CNpyr], which is believed to be due to increased CO₂ physicalsolubility with increasing pressure.

The data of FIGS. 7-9 were used to obtain the Henry's law constant (H)and reaction equilibrium constant (k₁) for the reaction at varioustemperatures. These are plotted in FIG. 10. Based upon this information,the chemical reaction enthalpy this reaction is believed to be about 43kJ mol⁻¹ and the entropy is believed to be about 130 J mol⁻¹ K⁻¹.Differential scanning calorimetry gave a reaction enthalpy of about −53kJ mol⁻¹, which agrees with the data obtained from the isotherms.

Example 6

Trihexyl(tetradecyl)phosphonium 3-(trifluoromethyl)pyrazolide[P₆₆₆₁₄][2-CF₃-pyra] was prepared using a procedure described inExample 1. The viscosity data for this ionic liquid is also included inFIG. 8. FIG. 11 compares the isotherms of [P₆₆₆₁₄][2-CNpyr] and[P₆₆₆₁₄][2-CF₃-pyra] at 22° C. The reaction enthalpy of CO₂ with[2-CF₃-pyra] was determined to be about −46 kJ mol⁻¹ by calorimetry.

It is believed that similar viscosity properties will be observed forother ionic liquids described herein. This may make the ionic liquidsdescribed herein much more attractive for commercial applications.

While the above detail description has shown, described, and pointed outnovel features of the invention as applied to various embodiments, itwill be understood that various omissions, substitutions, and changes inthe form and details of the systems, methods, processes, or compositionsillustrated may be made by those skilled in the art without departingfrom the spirit of the invention. As will be recognized, the presentinvention may be embodied within a form that does not provide all of thefeatures and benefits set forth herein, as some features may be used orpracticed separated from others.

What is claimed is:
 1. An ionic liquid comprising an anion representedby a formula:

wherein X is N; A is N or CR³; E is N or CR⁴; R¹, R², R³, and R⁴ areeach independently H, halo, CN, CNO, NCO, NO₂, R¹¹, OR¹¹, SR¹¹, NR¹²R¹³,—YC(O)ZR¹¹, SO₂R¹¹, SO₃R¹¹, or SO₂NR¹²R¹³; Y is a single bond,optionally substituted C₁₋₆ hydrocarbyl, —N(R¹²)—, O, or S; Z is asingle bond, —N(R¹²)—, O, or S; each R¹¹ is H or optionally substitutedC₁₋₁₂ hydrocarbyl; and each R¹² and each R¹³ is independently H oroptionally substituted C₁₋₆ hydrocarbyl.
 2. The ionic liquid of claim 1,wherein R¹ and R⁴ are H and R² is NO₂.
 3. A method of removing carbondioxide from a mixture of gases, comprising: providing a mixture ofgases comprising carbon dioxide; and allowing sufficient contact betweenthe mixture of gases and a first ionic liquid comprising an anionaccording to the formula

wherein: X is N; A is N or CR³; E is N or CR⁴; and R¹, R², R³, and R⁴are each independently H, halo, CN, CNO, NCO, NO₂, R¹¹, OR¹¹, SR¹¹,NR¹²R¹³, —YC(O)ZR¹¹, SO₂R¹¹, SO₃R¹¹, or SO₂NR¹²R¹³; Y is a single bond,optionally substituted C₁₋₆ hydrocarbyl, —N(R¹²)—, O, or S; Z is asingle bond, —N(R¹²)—, O, or S; each R¹¹ is H or optionally substitutedC₁₋₁₂ hydrocarbyl; and each R¹² and each R¹³ is independently H oroptionally substituted C₁₋₆ hydrocarbyl; such that the carbon dioxidereacts with the first ionic liquid to form a second ionic liquidaccording to claim 1, thereby reducing the amount of carbon dioxide inthe mixture of gases.
 4. The method of claim 3, wherein the mixture ofgases is an exhaust from combustion of a carbon-based fuel, wherein thecarbon-based fuel is coal, a petroleum product, or natural gas.
 5. Themethod of claim 3, wherein the mixture of gases further comprises anoxide of sulfur or nitrogen.
 6. The method of claim 3, furthercomprising recovering the carbon dioxide from the second ionic liquid byapplying at least one of heat or reduced pressure to the second ionicliquid.
 7. A gas separation system comprising: a flow componentconfigured to provide a flow of a gas mixture, wherein the gas mixturecomprises carbon dioxide; and a separation component, in fluidcommunication with the flow component, configured to allow the flow ofthe gas mixture to pass through the separation component from the flowcomponent, wherein a quantity of a first ionic liquid is containedwithin or coupled to the separation component, wherein the first ionicliquid comprises an anion according to the formula

wherein: X is N; A is N or CR³; E is N or CR⁴; R¹, R², R³, and R⁴ areeach independently H, halo, CN, CNO, NCO, NO₂, R¹¹, OR¹¹, SR¹¹, NR¹²R¹³;—YC(O)ZR¹¹, SO₂R¹¹, SO₃R¹¹, or SO₂NR¹²R¹³; Y is a single bond,optionally substituted C₁₋₆ hydrocarbyl, —N(R¹²)—, O, or S; Z is asingle bond, —N(R¹²)—, O, or S; each R¹¹ is H or optionally substitutedC₁₋₁₂ hydrocarbyl; and each R¹² and each R¹³ is independently H oroptionally substituted C₁₋₆ hydrocarbyl; and wherein the system isconfigured to provide sufficient contact between the gas mixture and thefirst ionic liquid to remove at least a portion of the carbon dioxidefrom the gas mixture to form an ionic liquid according to claim 1; andwherein the system contains the ionic liquid according to claim
 1. 8.The gas separation system of claim 7, further comprising a regenerationcomponent wherein the ionic liquid according to claim 1 is subjected toheat and/or reduced pressure to liberate carbon dioxide.
 9. A combustiondevice comprising: a combustion vessel configured to contain acombustion reaction; an exhaust component, in fluid communication withthe combustion vessel, which is configured to allow exhaust from thecombustion reaction to escape from the combustion vessel, wherein theexhaust comprises carbon dioxide; and the gas separation system of claim7, coupled to the exhaust component; wherein the device is configured toprovide sufficient contact between the exhaust and the separation systemto remove at least a portion of the carbon dioxide from the exhaust.